We all have enough to worry about when we're on our periods—like aching pains, leaking tampons, and mood swings, for starters. "Getting pregnant" doesn't typically make the list, but if you're a fan of period sex, it's something you should actually be thinking about before jumping into bed (or the shower).

"It is completely possible to conceive during your period if you are not using protection and not on any birth control," Melissa Walsh, M.D., an ob/gyn in the department of obstetrics, gynecology, and women’s health at Montefiore Medical Center, tells SELF. The likelihood really depends on the length of your cycle.

The first day of your period is considered day one of your cycle. "A woman with regular monthly periods typically ovulates (releases an egg) on day 14 or 15 of her cycle, and is able to conceive [if she has sex] five days prior to and one day after she ovulates," Walsh explains. That’s because sperm can last up to five days in the cervical mucous and uterus. But some women have shorter cycles and, therefore, ovulate earlier. If that's you, and you bleed for six or seven days and have unprotected sex on one of the last days of your period, it's possible that you can conceive.

The least likely time for conception during your period is the first two to three days, "but even this reduced risk for pregnancy is highly dependent on whether or not you have regular periods or not," she adds. If your cycle is irregular and changes from month-to-month, it might be tough to ever accurately predict when you'll be ovulating.

If you're on the Pill and take it religiously, your chances of getting pregnant while bleeding are much lower than if you weren't on birth control because (assuming you’re on the popular combo pill) it prevents the release of an egg in the first place. But, Walsh says, "No birth control is 100 percent effective, not even getting your tubes tied!" And if even that might not stop you from getting pregnant, then we don’t recommend relying on your period to.